Lewis acidity of methyltrioxorhenium(VII) (MTO) based on the relative binding strengths of N-donors

Article abstract of DOI:10.1021/ja056505c

This article presents a σ acceptor strength scale for methyltrioxorhenium(VII) (MTO), one of the most versatile and useful high oxidation state organometallics ever described, The spectrophotometric titration of MTO with a series of N-donor bases in CCl₄ gives formation constants (K_I) and enthalpies for the adduct formation reactions. An excellent linearity of log K_I with respect to the Hammett σ constants of the substituents on the ligands was observed. The resulting ρ constant is proposed to be a good indication of the Lewis acidity of MTO. The enthalpies of adduct formation of N-donors with MTO also fit the ECW model to predict the values of E_A and C_A parameters for MTO, The parameters can be used to predict an acidity scale for MTO. These parameters also allow the chemists to predict and correlate quantitatively the enthalpies of MTO·Lewis base interactions. Significant chemical insights result from the fit of the data to the ECW model.

Full text of DOI:10.1021/ja056505c

Published on Web 12/14/2005
Lewis Acidity of Methyltrioxorhenium(VII) (MTO) Based on the
Relative Binding Strengths of N-Donors
S. Masoud Nabavizadeh* and Mehdi Rashidi
Contribution from the Department of Chemistry, College of Sciences,
Shiraz UniVersity, Shiraz 71454, Iran
Received September 22, 2005; E-mail: nabavi@chem.susc.ac.ir
Abstract: This article presents a σ acceptor strength scale for methyltrioxorhenium(VII) (MTO), one of the
most versatile and useful high oxidation state organometallics ever described. The spectrophotometric
titration of MTO with a series of N-donor bases in CCl4 gives formation constants (Kf) and enthalpies for
the adduct formation reactions. An excellent linearity of log Kf with respect to the Hammett σ constants of
the substituents on the ligands was observed. The resulting F constant is proposed to be a good indication
of the Lewis acidity of MTO. The enthalpies of adduct formation of N-donors with MTO also fit the ECW
model to predict the values of EA and CA parameters for MTO. The parameters can be used to predict an
acidity scale for MTO. These parameters also allow the chemists to predict and correlate quantitatively the
enthalpies of MTO‚Lewis base interactions. Significant chemical insights result from the fit of the data to
the ECW model.
compounds,1-5,18-22 Baeyer-Villiger oxidation and Dakin
Introduction
reaction,3-5,23-25 oxidation of sulfur compounds,1-5,26-30 oxida-
Methyltrioxorhenium(VII) (MeReO3 or MTO), a superstar
compound, has been known for more than 2 decades. For a
considerable part of this time, it has been widely regarded as a
mere curiosity. This picture changed dramatically during the
past decade. Today, many derivatives of MTO are known and
easily accessible. Several of these compounds, most notably
MTO itself, have found numerous very interesting applications
in both catalysis and material science. The numerous different
processes, in which MTO can be applied as a catalyst, are
summarized as follows: oxidation of alkenes,1-11 oxidation of
conjugated dienes,12 epoxidation of allylic alcohols and 1,3-
transposition of allylic alcohols,3-5,13-17 oxidation of aromatic
tion of phosphines, arsines, and stibines,3-5,31 oxidation of ani-
lines and amines,3-5,32 oxidative cleavage of N,N′-dimeth-
ylhydrazones,3-5,33-35 oxidation of halide ions,1-5,36,37 oxidation
of C-H and Si-H bonds,1-5,38,39 oxidation of metal carbon-
yls,40,41 aldehyde olefination, and related reactions,1-5,8,42-47
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9
10.1021/ja056505c CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 351-357
351

A R T I C L E S
Nabavizadeh and Rashidi
Scheme 1. Reaction of MTO with hydrogen peroxide
chemistry. The 2- and 2,6-substituted derivatives invariably have
steric problems with Lewis acids. Pyridines substituted in the
3- and 4-positions provide a family of donors with a constant
steric requirement toward bulky Lewis acids. Since steric
problems are often encountered in transition metal ion chemistry,
the pyridine family is often used to determine electronic
influences on the chemistry. A recent review on the role of
nitrogen ligands in homogeneous catalysis57 states the bond
Scheme 2. Lewis base adducts of MTO
dissociation energies of the adducts of some aliphatic amines
61-63
with BMe3
as the only quantitative data relevant to the
coordination strength of nitrogen ligands, but these data are
hardly of any use to predict the strength of the interaction
between the usually employed ligands (most of which contain
sp2-hybridized nitrogen atoms) and a transition metal. Several
papers, especially in the older literature, have been devoted to
phenanthroline and bipyridine complexes,64,65 but most of the
available data were measured in aqueous solution, where
protonation of the ligand becomes important. Thus, this
information cannot be assigned to the low-to-medium polarity,
often anhydrous, solvents commonly employed for catalytic
processes.
olefin metathesis,8,44-47 and Diels-Alder reaction.1-5,48 Despite
the important applications of MTO, it is surprising that no
quantitative information exists on the acidity strength of this
important compound.
In the case of olefin epoxidation, full studies were made about
the nature of the catalytically active species by using H2O2 as
oxidizing agent. MTO reacts with H2O2 (see Scheme 1) to form
mono- and bisperoxo complexes, depending on the amount of
H2O2 used.49
Considering the important applications of MTO and its Lewis
base adducts (compounds 1 and 2 in Scheme 2) in catalytic
activity, a field where the Lewis acidity of MTO is well-known
to play an important role, we have measured and extended the
relative coordination ability of different N-donor ligands (shown
in Scheme 3) toward MTO. The data obtained are used to
present a scale of σ acceptor strength for MTO. We have also
measured the enthalpies of the adduct formation in the poorly
solvating solvent, CCl4. The Drago’s EA and CA parameters for
MTO are also obtained. This is particularly significant because
of the importance of MTO in inorganic, organometallic, and
catalytic chemistry. These parameters also allow the chemists
to predict and correlate quantitatively the enthalpies of MTO‚
Lewis base interactions.
The most important disadvantage of the MTO/H2O2 system
is the parallel formation of diols instead of the desired epoxides,
especially in the case of more sensitive substrates.50 It was
quickly detected that the use of Lewis base adducts of MTO
(compounds 1 and 2 in Scheme 2) significantly decreases the
formation of diols due to the reduced Lewis acidity of the
catalytic system.10
In recent years, the catalytic activities of the Lewis base
adduct of MTO have been studied. MTO forms trigonal
bipyramidal adducts with monodentate N-bases and (dis-
torted) octahedral adducts with bidentate Lewis bases.7,51-56
The monodentate Lewis base adducts of MTO react with H2O2
to form mono- and bisperoxo complexes analogous to that of
MTO, but coordinated by one Lewis base molecule instead of
H2O (compound 3 in Scheme 2). The activity of the bisperoxo
complexes in olefin epoxidation depends on the Lewis bases,
the redox stability of the ligands, and the excess of Lewis base
used. The peroxo complexes of the MTO Lewis bases are, in
general, more sensitive to water than MTO itself.8,9
Experimental Section
The compound methyltrioxorhenium (MeReO3 or MTO) was
prepared as reported.66 The N-donor ligands (generically designated
L) were purchased from commercial sources (Merck or Fluka) and were
used as received. CCl4 (Merck, quality; for spectroscopy) was used as
solvent throughout this study, and the data were obtained at different
temperatures. Equilibrium studies were carried out by using a Perkin
Elmer Lambda 25 spectrophotometer with temperature control using
an EYELA NCB-3100 constant temperature bath. Typically, a 2 ×
10-4 M solution of the MTO in CCl4 contained in a quartz cuvette
with a 1 cm path length was treated with successive aliquots of a
solution of the N-donor ligand, of known concentration, in the same
solvent. The values of formation constants (Kf) were determined by
On the other hand, nitrogen ligands are of great impor-
tance in homogeneous catalysis.57-60 The substituted pyridines
are also a set of ligands commonly used in coordination
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9
352 J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006

Lewis Acidity of Methyltrioxorhenium
A R T I C L E S
Scheme 3. Structural formulas of N-donor ligands
fitting the equilibrium absorbance to eq 1 by the method of nonlinear
least squares.53
relationships were observed between the formation constants
and either the Hammett σ constants (see Figure 1) or the pKa
values of the ligands as log Kf ) m(pKa) + b (see Figure 2).
The values for the F constant, the slope of the log Kf/ligand’s
pKa plot (hereafter referred to as m), and the corresponding R2
(correlation coefficient) values are collected in Table 2.
Very recently, Ragaini et al.,73 for the first time, have pointed
out that F can be considered as a measure of the Lewis acidity
of the metal complexes bearing completely different ligands and
having metals with different oxidation states. They have also
stated that in the literature very little experimental quantitative
information exists on the Lewis acidity of transition metal
complexes. In this paper, we attempt to make a comparison
between the F constants of different chemicals, including MTO
(see Table 2). A series of formation constants has been measured
for determination of the F constant. The data presented can be
employed to give an experimental answer to an important
question, that is, the acidity of MTO.
ꢀΜΤÃ + ꢀMTO‚L Kf[L]
(Absorbance)eq
)
[MTO]0
(1)
{
}
1 + Kf[L]
Here, (Absorbance)eq is the absorbance of a solution at equilibrium;
ꢀMTO and ꢀMTO‚L are molar absorptivities for MTO and MTO‚L species,
respectively at corresponding wavelength; [MTO]0 is the total concen-
tration of MTO and MTO‚L in the solution, and [L] represents the
equilibrium ligand concentration.
Result and Discussion
Formation Constants and Linear Free-Energy Relation-
ship (LFER). Very recently,53-56 we have studied the interaction
of ΜΤÃ and different N-donor ligands (eqs 2 and 3) in CHCl3
and benzene.
One important observation that is clear from the data in Tables
1 and 2 is that, although more basic ligands coordinate more
strongly in all cases, the slope of the line describing the
correlation is highly variable (F ranges from -1.0 to -5.6).
The magnitude of the reaction constant, F, is essentially a
measure of the susceptibility of a reaction to the polar effect of
substituents in the adjacent benzene ringsthe larger the value
of F, the greater the susceptibility of the reaction. The sign of
F is also of diagnostic use in that a negative F value indicates
the development of a positive charge (or, of course, the
disappearance of the negative charge) at the reaction center,
In this work, for reaction 2 in CCl4 (a poorly solvating solvent
to minimize the solvation effect), the program PSEQUAD67 or
KaleidaGraph 3.6 was used to determine Kf by a global fit of
absorbance-concentration data taken at 8 wavelengths in the
range of 300-400 nm. The values of Kf so calculated are given
in Table 1. The corresponding Hammett σ value and the pKa
value of the ligands are also shown. Where two substituents
were present, the sum of the individual σ constants is reported
and was employed in the following fitting. The excellent linear
(68) Nelson, D. W.; Gypser, A.; Ho, P. T.; Kolb, H. C.; Kondo, T.; Kwong,
H.-L.; McGrath, D. V.; Rubin, A. E.; Norrby, P.-O.; Gable, K. P.; Sharpless,
K. B. J. Am. Chem. Soc. 1997, 119, 1840.
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A 2002, 58, 2563.
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chimica Acta Part A 2002, 58, 1375.
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J.; Panson, G. S. J. Phys. Chem. 1965, 69, 3089. (c) Kai, F.; Haraga, H.;
Kobayashi, H. Polyhedron 1985, 4, 947.
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81st ed.; CRC Press: Boca Raton, FL, 2000.
(73) Gasperini, M.; Ragaini, F. Organometallics 2004, 23, 995.
(67) Ze´ka´ny, L.; Nagypa´l, I. In Computational methods for the determination
of formation constants; Leggett, D.; Plenum Press: New York, 1985. We
are grateful to Dr. Ga´bor Lente for assistance in the use of the program.
9
J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006 353

A R T I C L E S
Nabavizadeh and Rashidi
Table 1. Formation Constants (Kf) for the Adduct Formation Reactions (eq 2) between MTO and Different N-Donor Ligands (shown in
Scheme 3) and ∆H° Values in CCl4
1
Kf/L mol-
a
1
ligand
10
°
C
15
°
C
20
°
C
25
°
C
30
°
C
35
°
C
40
°
C
pKa
σ b
∆H°
/kJ mol-
1
2
3
4
5
6
7
8
9
10
11
12
13
14
8619.9 ( 929.9 6487.8 ( 436.1 5080.4 ( 166.8 3947.3 ( 148.9 3128.2 ( 117.3 2589.4 ( 95.1 2144.9 ( 56.5 6.99
2.49
9723.6 ( 470.5 7307.7 ( 352.2 5736.5 ( 298.9 4444.5 ( 185.0 3526.6 ( 97.6 2935.3 ( 85.9 2430.6 ( 56.4 7.11
11110.0 ( 409.3 8440.7 ( 333.3 6330.0 ( 380.9 4890.9 ( 212.1 3601.2 ( 110.7 3000.9 ( 109.8 2410.6 ( 104.4 7.09
c
c
c
c
0
-35.4 ( 0.6
-38.9 ( 1.6
-35.3 ( 0.7
-38.6 ( 0.8
-34.8 ( 0.2
105.3 ( 6.6
84.3 ( 3.6
60.5 ( 2.9
48.9 ( 3.0
37.8 ( 1.7
25.1 ( 1.9
19.8 ( 1.5
1500.9 ( 56.0 1175.6 ( 32.8
908.0 ( 13.5
720.7 ( 11.9
569.1 ( 64.1
450.6 ( 9.1
360.2 ( 12.0 5.23
3422.1 ( 157.7 2594.0 ( 138.6 1936.7 ( 56.6 1544.3 ( 68.0 1247.8 ( 90.4
996.9 ( 35.8
819.8 ( 27.9 5.98 -0.2 -36.2 ( 0.7
5887.0 ( 269.3 4412.7 ( 198.6 3396.6 ( 119.5 2649.6 ( 99.6 2103.9 ( 91.7 1700.7 ( 85.8 1314.8 ( 66.5 6.59 -0.27 -36.7 ( 0.4
1959.8 ( 101.8 1519.9 ( 96.4 1155.3 ( 46.4
914.0 ( 24.0
726.8 ( 54.8
562.2 ( 22.7
870.0 ( 18.6
42.9 ( 1.6
280.2 ( 21.3
48.9 ( 2.8
464.6 ( 12.9 5.52 -0.07 -35.8 ( 0.3
719.3 ( 16.4 5.94 -0.17 -36.2 ( 0.4
3038.8 ( 135.8 2301.3 ( 111.4 1764.2 ( 91.1 1388.0 ( 69.8 1103.1 ( 70.0
130.6 ( 5.8
948.0 ( 50.0
160.9 ( 8.4
100.1 ( 3.9
719.3 ( 46.3
130.1 ( 7.9
80.0 ( 4.5
588.1 ( 29.0
103.9 ( 9.0
67.1 ( 5.2
469.4 ( 23.7
84.8 ( 6.9
55.6 ( 2.9
373.6 ( 9.5
67.7 ( 3.8
36.0 ( 2.0
2.95 0.37 -31.4 ( 0.8
231.3 ( 15.8 4.85 0.06 -33.9 ( 0.8
39.6 ( 2.4 3.13 0.32 -32.6 ( 1.2
5199.3 ( 347.0 3945.9 ( 175.6 2979.5 ( 115.2 2328.2 ( 107.9 1849.0 ( 102.0 1493.6 ( 49.3 1135.7 ( 57.6 6.46 -0.24 -37.1 ( 0.4
2102.9 ( 144.0 1628.7 ( 123.9 1249.8 ( 99.4
979.9 ( 62.4
779.2 ( 35.9
621.7 ( 27.8
499.7 ( 29.4 5.59
c
-35.5 ( 0.2
a Ka applies to the reaction HL+ ) H+ + L in aqueous solution. b The σm parameter was used for substitution at C(3), and the σp parameter was used for
substitution at C(4). The sum of the individual σ values is reported when two substituents are present on the pyridine ring. c Not defined.
Table 2. Values of F (reaction constant) and m (slope of the
correlation log Kf versus N-donor pKa Values) for Different
Chemicals
Chemicalsa
F
R2
(
F)
m
R2 (m)
solvent
ref
MTO
MTO
OsO4
C60-fullerene
o-Chloranil
Zn(TPP)
Zn(OEP)
Phenol
-2.5 0.996 0.43 0.999 CCl4
this work
-2.4 0.987 0.41 0.992 benzene 55
-1.9 0.991 0.34 0.996 toluene
-2.9 0.997 0.71 0.997 CCl4
-3.2 0.993 0.78 0.993 CCl4
-2.7 0.992 0.65 0.993 toluene
-2.0 0.987 0.49 0.987 toluene
-1.0 0.997 0.18 0.986 CCl4
68
69
70
71a
71a
71b
71c
72
[Cu(PhMePh)2] -1.7 0.998 0.34 0.980 CHCl3
H+
-5.6 0.996 1.00 1.000 water
a o-Chloranil)3,4,5,6-Tetrachloro-1,2-benzoquinone, TPP)tetraphenylpor-
phyrin, OEP)octaethylporphyrin, PhMePh)1-hydroxy-3-phenyl-4,5-di-
methylpyrazole 2-oxide
Figure 1. Plot of log(Kf/K0) at 298 K for the adduct formation reaction of
MTO with N-donors as a function of the Hammett σ constants for the
substituents on the pyridine ring. Data are from Table 1.
has been used and is shown in Table 2. Note that the solvents
used in the list (i.e., CCl4, benzene, and toluene) are different,
but their dielectric constants have close values. Clearly, acidity
order is as follow:
H+ > o-chloranil > C60-fullerene > Zn(TPP) > MTO >
Zn(OEP) > OsO4 > [Cu(PhMePh)2] > phenol
Thus, MTO has a greater Lewis acidity than OsO4 toward
pyridine derivatives.
Enthalpy Analysis with the ECW Model. In this section,
we try to establish another scale for acidity of MTO. The
formation constants (Kf) for reaction 2 with N-donors (L) were
evaluated as a function of temperature over the range of 283-
313 K. The enthalpy values were evaluated from the corre-
sponding temperature data by applying a nonlinear Kf least-
squares analysis according to the eq 4 (see Figure 3). The
enthalpy values are listed in Table 1.
Figure 2. Plot of log Kf (at 298 K) for the adduct formation reaction of
MTO with N-donors as a function of the pKa of the N-donors. Data are
from Table 1.
while vice versa, a positive F value indicates the development
of a negative charge (or the disappearance of the positive charge)
at reaction center.
-∆H°
RT
∆S°
R
Kf ) exp
× exp
(4)
(
)
(
)
The negative reaction constant for the pyridine equilibria with
acids (Table 2) shows that a positive charge expands on the
pyridine nitrogen in the adduct as compared with the free ligand.
As expected, the metal center acts as an electron acceptor and
attracts electrons upon coordination. To compare the Lewis
acidity of MTO, the Hammett correlation for other chemicals
Chemists have long been interested in the interaction of acids
with bases. To predict whether the magnitude of such an
interaction will be large or small, acids and bases have
traditionally been ranked in sequence according to their strength.
Coordination chemists have recognized that there is no single
order of acid strength, but rather the relative order within a group
9
354 J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006

Lewis Acidity of Methyltrioxorhenium
A R T I C L E S
Table 3. Selected Parameters of the ECW Modela
Base
EB
CB
Acid
EA
CA
1-Methylimidazole
Pyridine
1.16
1.78
1.81
1.66
1.83
1.83
1.86
1.84
1.75
1.70
1.80
4.92
3.54
3.67
3.08
3.73
3.83
3.86
3.70
3.41
3.24
3.65
I2
H2O
BMe3
BF3
0.50
1.31
2.90
6.10
0.51
2.72
2.27
2.87
1.82
1.60
3.15
2.00
0.78
3.60
2.87
1.56
1.45
1.07
0.71
2.86
1.96
1.05
3-Methylpyridine
3-Chloropyridine
4-Methylpyridine
4-Methoxypyridine
3,4-Dimethylpyridine
4-tButylpyridine
3-Phenylpyridine
Methylnicotinate
4-Benzylpyridine
SO2
Zn(TPP)b
Phenol
Me3SnCl
Cu(HFacac)2
c
d
VO(acac)2
Mo2(PFB)4
e
a Data from ref 77. Units for EB, CB, EA and CA are (kcal/mol)1/2
.
b TPP)tetraphenylporphyrin. c HFacac)hexafluoroacetylacetonato. d Van-
adyl acetylacetonate, data from ref 80. e PFB)perfluorobutyrate.
Figure 3. Plot of Kf versus T for adduct formation of MTO with N-donor
ligands [(a) 1-butylimidazole; (b) imidazole; (c) 3,4-dimethylpyridine; (d)
4-tert-butylpyridine; (e) pyridine] in CCl4.
of acids varies with the base being studied. The ECW approach
has been successfully employed74-77 for this case and to
correlate many neutral base-neutral acid adduct formation
enthalpies to the eq 5.
-∆H ) EAEB + CACB + W
(5)
EAEB and CACB describe electrostatic and covalent contribu-
tions for the donor (B)-acceptor (A) reaction. The W term is
zero for simple donor-acceptor adduct reactions in poorly
solvating media (e.g., CCl4), but for a more complex reaction,
it incorporates any constant contributions of a particular acid
(or base) that is independent of the base (or acid) it reacts with.
Equation 5 has, for the most part, been applied to 1:1 adduct
formation enthalpies.
Figure 4. A plot of eq 6 for a series of N-donors (the inset: for pyridine
derivatives only) reacting with MTO. MTO’s EA and CA parameters from
the slope and intercept of the plot are calculated as 2.38 (kcal/mol)1/2 and
1.15 (kcal/mol)1/2, respectively.
The most modern inorganic textbooks have also discussed
the ECW model.78 This approach76,77 is the successful attempt
at obtaining a quantitative scale of bond strengths that uses
donor-acceptor enthalpies of formation measured in poorly
solvating solvent. On the basis of a large database of E and C
parameters, chemists and biochemists can predict and correlate
quantitatively the enthalpies of Lewis acid-base interactions.
In the present study, the values for EA and CA parameters of
MTO are obtained. Thus we have used a graphical representation
of the E and C equation that can result for MTO when the base
is varied. Factoring and rearranging eq 5 (with W ) 0) leads
to79
The value of -∆H (in kcal/mol) divided by (CB + EB) for
N-donors is plotted versus the value of (CB - EB)/(CB
+
EB) calculated for N-donors from their parameters in Table 3
(see Figure 4). The data fit to eq 6 gives values of EA ) 2.38
(kcal/mol)1/2and CA ) 1.15 (kcal/mol)1/2 for MTO. It has been
emphasized that in using eq 5 to solve for the unknown EA and
CA, one should use donors with different CB/EB ratios. For this
reason, we used a family of imidazoles (ligands 1-4 in Scheme
3) to extend this ratio, although the values of CB and EB are
only known for 1-methylimidazole. The reactions of MTO with
most of the donors (i.e. S- and P-donors) in the E and C
correlation have not been included in this work because of the
complexity of their interactions with MTO.1 Even though the
CB/EB ratios for ligands used in this study do not change
appreciably and provide only tentative EA and CA values for
MTO, the results are particularly important as they present an
acidity scale for MTO. Another very important use of these
parameters is the calculation of enthalpy of MTO interactions
for hundred of systems which have not been examined
experimentally.
CA + EA
CA - EA CB - EB
-∆H
CB + EB
)
+
(6)
(
)
(
)(
)
2
2
CB + EB
(74) Drago, R. S. Struct. Bonding 1973, 15, 73.
(75) Drago, R. S. Coord. Chem. ReV. 1980, 33, 251.
(76) Drago, R. S. Applications of Electrostatic-CoValent Models in Chemistry;
Surfside Scientific Publishers: Gainesville, FL, 1994, and references therein.
(77) Vogel, G. C.; Drago, R. S. J. Chem. Educ. 1996, 73, 701.
(78) (a) Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, Prentice-Hall:
Englewood Cliffs, NJ, 1991, (b) Huheey, J. E.; Keiter, E. A.; Keiter, R. L.
Inorganic Chemistry, Principles of Structure and ReactiVity, Harper-Collin,
New York, 1993, (c) Shriver, D. F.; Atkins, P. W., Inorganic Chemistry,
Oxford University Press: London, 1999, (d) Cotton, F. A.; Wilkinson, G.;
Gaus, P. L. Basic Inorganic Chemistry, John Wiley and Sons: New York,
1987, (e) Douglas, B. E.; McDaniel, D.; Alexander, J. Concepts and Models
of Inorganic Chemistry, Wiley: New York, 1994.
To put the acidity of MTO on an external scale, it is necessary
to make comparisons with other well-known acids; I2, H2O,
BMe3, BF3, C6H5OH, SO2, Me3SnCl, Zn(TPP) {TPP)tetraphenyl-
porphyrin}, Cu(HFacac)2 {HFacac)hexafluoroacetylacetonato},
VO(acac)2 {vanadyl acetylacetonate} and Mo2(PFB)4 {PFB)per-
(79) Cramer, R. E.; Bopp, T. T. J. Chem. Educ. 1977, 54, 612.
9
J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006 355

A R T I C L E S
Nabavizadeh and Rashidi
like pyridine (CB)3.54, EB)1.78 and (CB -EB)/(CB+EB))0.33;
the dashed line a in Figure 5) the acid strength order is
Cu(HFacac)2 > VO(acac)2 > Mo2(PFB)4 >
MTO > I2 > Me3SnCl >SO2
For a base like Me2Se (CB)4.24, EB)0.05 and (CB -EB)/
(CB+EB))0.98; the dashed line b) the order is
Cu(HFacac)2 > I2 > VO(acac)2 > SO2 >
MTO > Mo2(PFB)4 > Me3SnCl
and a base like CH3CN (CB)0.71, EB)1.64 and (CB -EB)/
(CB+EB)) -0.4; the dashed line c) produces an acid strength
order
Mo2(PFB)4 > Me3SnCl > Cu(HFacac)2 >
MTO > VO(acac)2 > I2 > SO2
Therefore it is clear that in order for the lines for two acids
to cross, a necessary and sufficient condition is that a direct
comparison of their EA or CA numbers yield opposite relative
ordering.
Figure 5. The E and C plot of the various acceptor orders for representative
Lewis acids.
fluorobutyrate} were chosen for this purpose. EA and CA
parameters of these acids are given in Table 3.
Finally, it should be noted that the ECW model will predict
the normal, sigma bond energy for adducts, including molecules
which undergo drastic changes in their geometry upon adduct
formation. When steric effect has an important role, systems
are found that do not fit ECW model.75,81 Acids such as BMe3,
BF3, Me3SnCl and MTO undergo extensive rearrangement from
their structure as free acid when they form adducts. It is
shown75,81 that acids such as BMe3, AlMe3 and Me3SnCl may
encounter steric repulsion with certain bases such as (CH3)3N
and (C2H5)3N. However excellent agreement between experi-
mental enthalpies of adduct formation and the calculated one
using ECW model, for these Lewis acids with pyridine
derivatives indicates that steric effects are minimized with the
N-donors selected for incorporation into our study in this work.
It is reported77 that if both EA and CA are larger for one acid
than another, the first acid is stronger toward any base. The
difficulty arises if the comparison of CA number yields the
opposite rank order from the comparison of EA number. Cramer
and Bopp reported79 a way in which the E and C numbers for
a series of acids (or bases) could be plotted to facilitate such
comparative discussion. Using the reported EB and CB param-
eters from Table 3 and eq 5, -∆H is calculated for an acid of
interest; for example I2 interacting with some bases, listed in
Table 3. The values of -∆H (in kcal/mol) divided by (CB+EB)
for bases are plotted versus the values of (CB -EB)/(CB+EB)
calculated from their parameters in Table 3. Equation 6 is that
of a straight line and the calculated enthalpies for all bases
interacting with this acid will fall on this line. The procedure is
repeated for a series of acids (listed in Table 3) including MTO
and the resulting plots are shown in Figure 5. The acceptor order
toward any base whose EB and CB values are known can be
determined from the corresponding plot.
Conclusion
The results of this work can be summarized as follow:
a) In this work we have measured the relative coordination
ability of different N-donor ligands toward MTO. A linear
correlation between the relative coordination strength and either
the Hammett σ constant of the substituents on the ligands or
the pKa of the N-donor was always found. This allowed one to
measure the Lewis acidity of MTO using reaction constant, F.
Clearly MTO is stronger acid than other metaloxo species OsO4
toward pyridine derivatives.
b) On the basis of ECW model and comparing EA and CA
parameters, obtained for MTO, with parameters for other well-
known acids, the following results can be obtained: 1) Since
both EA and CA values are greater for BMe3, BF3 and Zn(TPP)
as compared to those for MTO, they are the stronger acids
toward all bases. 2) Since both EA and CA values are greater for
MTO than those for H2O and phenol, MTO is the stronger acid
toward all bases. 3) In the case of MTO, SO2, Cu(HFacac)2,
VO(acac)2, Mo2(PFB)4, Me3SnCl and I2, the situation is
Figure 5 shows the relative ordering of the acids when a
common base is used. Different bases are located along the
horizontal axis depending on their value of (CB -EB)/(CB+EB).
Then a line parallel to vertical axis at this point is drawn (the
dashed lines in Figure 5). This line intersects the various acid
lines in the proper sequence. Due to the large number of
intersections of acid lines, it is clear that the ordering of acids
will change from base to base. As pointed out above and
graphically shown in Figure 5, BF3 is, for example, a stronger
acid than MTO with all bases; this is a direct result of the fact
that these acid lines do not cross. Similarly, MTO is a stronger
acid than phenol or H2O with all bases. When the acceptor lines
for two different acceptors cross, the order of acceptor strength
changes toward bases on opposite sides of the intersection,
which is the case for comparing of MTO with Cu(HFacac)2,
VO(acac)2, Mo2(PFB)4, I2, SO2 and Me3SnCl. Toward a base
(80) Drago, R. S. Polyhedron, 1994, 13, 2017.
(81) Drago, R. S.; Vogel, G. C.; Needham, T. E. J. Am. Chem. Soc. 1971, 93,
6014.
9
356 J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006

Lewis Acidity of Methyltrioxorhenium
A R T I C L E S
Table 4. MTO-Additive-Catalyzed Oxidation of Cyclooctene with
large enthalpy values tend to deactivate MTO and therefore
decrease the epoxide yield. The reported MTO-additive-
catalyzed oxidations of cyclooctene with hydrogen peroxide are
presented in Table 4. As is clear from this table, 4-cyanopyridine
is more efficient than 4-methylpyridine in epoxidation of
cyclooctene. Generally, an additive acts as a Lewis base and
reduces the acidity of MTO. This would prevents epoxide-ring
opening and confirms the utility of MTO‚N-donor in epoxida-
tion. Despite huge studies on the role of additives in these
systems, the effects of the basic additives are still a topic for
conjecture.82
Hydrogen Peroxidea
Additive
EB
CB
-
∆
H b
Yield % c
-
-
-
-
45
70
65
65
75
Pyridine
1.78
1.83
1.84
1.53
3.54
3.73
3.70
2.94
8.31
8.64
8.63
7.02
4-Methylpyridine
4-tButylpyridine
4-Cyanopyridine
a Data from ref 7; the reactions have been performed at room temperature
with a MTO/H2O2/cyclooctene ratio of 0.01:1.5:1.0 and the MTO/additive
ratio of 1:25. b ∆H (kcal/mol) is calculated using ∆H ) EMTO‚EB
CMTO‚CB. c Epoxide yield after 4 h.
+
ambiguous. Thus MTO is more acidic than I2 toward bases with
EB.CB, but is a weaker acid for bases where CB.EB.
Acknowledgment. The authors acknowledge the support of
the Shiraz University Research Council (Grant No. 84-GR-SC-
12) for this research. We are grateful to Mr. Alireza Akbari for
exploring the initial interactions between N-donors and MeReO3
and Mr. Yektaii and Dr. Razavizadeh for their great valuable
helps.
The scales, suggested in this work, for Lewis acidity of MTO
can be correlated to catalytic activity, a field where the Lewis
acidity of the metal is well-known to play an important role. In
MTO-catalyzed epoxidation reactions, the reactivity depends
on the strength of interaction of MTO with additive. The extent
of this interaction can be correlated to the enthalpy (i.e. the
bond strength of Re-N) of the reaction. N-donors involving
JA056505C
(82) Lane, B. S.; Burgess, K. Chem. ReV. 2003, 103, 2457.
9
J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006 357